It will be a lot of fun no matter what but probably will only work in sub planing conditions. The main issue will be the size of the fin and to a lesser extent it's position. If you can somehow jamb a larger fin in it it could be interesting..........

789 & Sailquik

Yeah..it's got me thinkin' too, the whole tandem thing.
Dunno why, but just looks & sounds like a bit of off the wall fun.

I have a 3.55m x .66m, 180lt, TC Spacer Nova, squash tail winger(!wow) double concave. Crosslinked polyethylene, foam filled.

Similar length to your board Andrew, so would put mast tracks in similar position etc. need to fill dagger board & somehow fit modern fin box. maybe just bolt on mast tracks???
Any ideas welcome...cheers